Water-cooled reactors, and particularly pressurized-water reactors, have a core made up of groups of fuel rods with each group positioned in a fuel rod and control rod guide tube assembly.
This assembly uses a fuel assembly support structure having the characteristics of a skeleton. It comprises top and bottom end pieces having openings for the control rod guide tubes and fuel rods, a plurality of fuel rod spacer grids having corresponding openings and the control rod guide tubes having top and bottom ends respectively fastened to the top and bottom end pieces and holding these pieces interspaced. The guide tubes are inserted slidably through the openings in the spacer grids and because the spacer grids must be held at interspaced positions between the end pieces, the spacer grids must be fixed to the control rod tubes, this also being necessary to make the fuel assembly support structure as rigid as possible.
Such a support structure may have a length in the order of 4,840 mm and have a cross section of only 230 mm .times. 230 mm in the case of a square cross section. The control rod guide tubes may have a diameter in the order of only about 12 mm, and since they are used for the control rods which control the reactivity of the reactor core, when the structure is loaded with the fuel rods to form the assembly in the reactor core under service conditions, the fuel assembly support structure rigidity, including the guide tubes, is important.
All of the parts are, of course, made of metals or metal alloys. Therefore, to fix the spacer grids and guide tubes together in a rigid manner, the prior art has used welding and soldering techniques involving the alloying of molten metal with the materials of the two parts to be fixed together.
However, for reasons of nuclear physics, the guide tubes and spacer grids should preferably be made of differing metals or alloys. For example, the guide tubes are desirably made of stainless steel while the spacer grids are desirably made of a zirconium alloy. Such differing materials cannot be joined together reliably by known techniques, such as by soldering or welding, involving the incidental use of solidified molten metal.
Therefore, although undesirable from the viewpoint of nuclear physics, the prior art has involved the use of spacer grids and guide tubes made of the same metal or alloy and joined together by spot welding, in many instances.